One use of self-drilling screws is to secure metal cladding to roof and wall structures. Such cladding is commonly of a corrugated or similar form in cross-section so as to have a plurality of alternating ridges and valleys. The crest of each ridge, and the base of each valley, may be curved or flat. Metal cladding of such kind may be secured to an underlying frame of metal or timber.
When a self-drilling screw is used to secure such metal cladding to a frame, it is generally arranged with its head bearing against the crest of a ridge of the cladding while the threaded end section of the screw is engaged within the underlying frame member. A section of the screw shank, which is not screw threaded, extends between the cladding ridge and the frame member. When installing the screw, the drill tip of the screw cuts a hole through the cladding so as to permit passage of the screw to the underlying frame member, and the drill tip subsequently serves to cut a hole within the frame member. The threaded section of the screw then forms a co-operating complementary thread within the sidewall portions of the frame member which define the hole and the screw is thereby fastened to the frame member.
A problem arises when the screw is to be used under high load situations or conditions and particularly in those situation which require the screw to have substantial resistance to corrosion. An example of such a situation is where the screw is used to fasten roof cladding to an underlying frame structure or member within a high wind region having a corrosive environment--such as, for example, a region located close to saltwater. In such cases, the corrosion resistance factor is generally created by providing the screw with a suitable coating --such as, for example, by galvanizing, electroplating, mechanical platting, or by means of the application of a barrier coating system. The integrity of any such coating is important with respect to the ability of the screw to resist corrosion.
It is quite common in screws of the foregoinq kind for the non-threaded shank section to have a diameter which is greater than that of the hole formed by means of the drill tip. Thus, when the screw is being installed, the shank needs to be forced through the hole defined within the cladding, and that tends to cause removal of the protective coating from portions of the screw shank. It will often be the case that such portions of the shank will remain exposed between the cladding and the frame member when the screw is finally fastened in position, and consequently will be subjected or exposed to the corrosive environment.
Such partial destruction of the protective coating can have serious consequences, and will not be readily observable in many cases. That is, there may be no knowledge or forewarning of the loss of the corrosion resistance until the screw fails under load conditions due to corrosion occurring at the damaged regions.